In conventional work machines, e.g., hydraulic excavators and wheel loaders, provided with an open center hydraulic circuit, the discharge flow rate from a hydraulic pump is controlled by means of the working hydraulic pressure in a center bypass. For example, Patent Reference 1 discloses a hydraulic circuit structure having an orifice (choke) provided in a center bypass, in which a negative control passage led from the upstream side of the orifice is communicating with a regulator control valve.
In this technique, the regulator control valve is controlled such that the discharge flow rate from a hydraulic pump is increased as the working hydraulic pressure in the negative control passage (i.e., the negative control pressure) decreases. It is considered that this construction can minimize the discharge flow rate from the hydraulic pump by introducing a higher negative control pressure to the regulator control valve when a hydraulic N cylinder or hydraulic motor on the circuit is not being operated (i.e., the lever is in the neutral position in the absence of any operation) or the magnitude of an operation (manipulated variable), if any, is very small. Such controls on the discharge flow rate from the hydraulic pump by means of the differential pressure of the orifice on the center bypass are generally referred to as “negative controls”.
The choking characteristic of an orifice used for negative controls is set, based on the pump characteristic for the discharge flow rate from the hydraulic pump when the work machine is in normal operation, i.e., while the engine is rotating at the rated engine speed. For example, the pump characteristic is set such that the discharge flow rate Q is increased as the negative control pressure Pn decreases and such that the discharge flow rate Q is reduced as the negative control pressure Pn increases, as indicated in the solid line in FIG. 3.
In this example, the pump characteristic at the rated engine speed is set such that the discharge flow rate Q is set to a first flow rate Q1 when the negative control pressure Pn is a first pressure P1 or greater, whereas the discharge flow rate Q is set to a second flow rate Q2 (Q2>Q1) when the negative control pressure Pn is smaller than a second pressure P2. In addition, in the range in which the negative control pressure Pn satisfies P2<=Pn<P1, the discharge flow rate Q is reduced in proportion to an increase in the negative control pressure Pn.
For such a pump characteristic, the choking characteristic of the orifice is set such that a negative control pressure that minimizes the discharge flow rate Q from the hydraulic pump is generated when the lever is in the neutral position. For example, as indicated in the broken line in FIG. 3, the choking characteristic of the orifice is set such that the negative control pressure Pn (i.e., the upstream pressure of the orifice) becomes the first pressure P1 or greater when the discharge flow rate Q is the first flow rate Q1. The pressure Pn1 corresponding to the point of intersection A of the solid line and the broken line in FIG. 3 is the negative control pressure when the lever is in the neutral position, and the pump flow rate corresponding to this point of intersection is the first flow rate Q1.